1. Field of the Invention
The present invention relates to a pretensioner which is used in a webbing take-up device, and which causes a webbing to be applied closely to a vehicle occupant when a vehicle suddenly decelerates.
2. Description of the Related Art
A seat belt device must insure the safety of a vehicle occupant by tightly restraining the vehicle occupant by a webbing when the vehicle suddenly decelerates.
The pretensioner is designed such that the webbing can be freely taken up or pulled out without any interference with the rotation of the take-up shaft during ordinary running of the vehicle. When the vehicle suddenly decelerates, the take-up rotary force of the pretensioner is transmitted to the take-up shaft of the webbing by a clutch mechanism provided between the take-up shaft and the pretensioner, so that the webbing is taken up in the direction which restrains the vehicle occupant.
The take-up rotary force of such a pretensioner is obtained in the following manner. A gas is fed under pressure into a cylinder so that a piston is caused to move suddenly, thus causing instantaneous rotation of a rotating drum around which a wire, one end of which is connected to the piston, is wound. The rotary force of the rotating drum is transmitted to the take-up shaft of the webbing by the clutch mechanism.
However, in conventional pretensioners equipped with a cylinder and piston, the cylinder must be lengthened in accordance with the amount off rotation of the take-up shaft. Accordingly, the cylinder portion protrudes from the main body of the webbing take-up device, so that a given attachment space must be provided inside the passenger compartment.